Hot water tanks may be provided in the form of pressure tanks (cf. U.S. Pat. No. 1,468,527). Such type of pressure sealed hot water tanks require a safety valve. In the prior known case, the safety valve is associated with the lid or cover of the hot water tank. Any failure of the safety valve will result in dangerous excess pressures. In addition, the variations in pressure can be only restricted in a very unsatisfactory manner with the aid of a safety valve and expensive and time-consuming measures will be necessary to avoid pressure variations at all. Variations in pressure in a hot water tank, though, will impair the dosing of the beverage dispensing machine the hot water tank is associated with.
Further counted among prior art are closed low-pressure systems in the case of which the top area is in communication with a separate expansion vessel wherein surplus water is accumulated. The expansion vessel, therefore, requires to be drained regularly. Hot water tanks of such type are expensive as to construction and, above all, require a lot of space which renders assembly into beverage dispensing machines difficult. To ensure accurate dosage they further require an expensive and time-consuming pressure or volume control.
In addition, low-pressure operated hot water tanks are known which are virtually open or are closed by means of a loose cover only so that pressure compensation may take place between the top area and the external atmosphere. No dangerous modes of operation will occur in such type of hot water tank. Accurate dosage will be possible with the aid of simple means, since variations in pressure normally will not have to be expected. The space requirement of such type of hot water tank is small as well, the hot water tank being operated at temperatures ranging between approximately 80.degree. and approximately 95.degree.. When the top area is covered or closed by means of the lid steam losses are kept low and the contents of the hot water tank is largely prevented from being contaminated. The top area of the hot water tank includes a water overflow port associated with a drain passage exteriorly secured to the tank wall. Upon failure in the liquid level control, surplus water may consequently be discharged. Moreover, in cases where the boiling point is reached due to, for instance, irregularities in temperature control, any excess pressures occurring as a consequence of the generated steam are eliminated. The steam--which is present also during normal operation--can escape via the external drain passage, condenses in said passage and, as dripping water, is then collected, for instance, by means of a dripping water collecting bowl. The collected dripping water is then again caused to evaporate from the collecting bowl by means of a ventilating device and/or an additional heating system.
The overflow edge of the overflow port or opening is spaced from the normal liquid level to allow the water reservoir in the hot water tank to expand accordingly upon heating without any water being lost. The overflow opening is so dimensioned that even when difficulties arise in supplying water, water will be prevented from penetrating into the cover region because electrical components are positioned in this region in most cases. This means that the overflow opening requires to be many times greater than is necessary for the pure pressure compensation between ambient atmosphere and top area of the hot water tank. The large dimension of the overflow opening has the effect that relatively great amounts of steam continuously escape--and this means, a correspondingly high loss in energy at the same time. In beverage dispensing machines of the type in question and of normal size, losses in condensation water may amount up to 300 ml and more per day.